An imaging spectrometer is an optical device for measuring the radiation from a radiant energy source as a function of wavelength. Conventional spectrometers consist of an objective lens or scan mirror which collects the radiation from the source and images it, by means of optical components onto the surface of a detector capable of converting the incident radiation into electrical signals.
Conventional spectrometers are used on photoreconnaissance satellites to photograph images from space. The scan mirror, which looks out into space from the satellite platform, rotates to scan and thus attain an image of the radiant energy source of interest. The scan mirror reflects this radiant energy onto the telescopic mirrors of the spectrometer from which the radiation is then passed through beamsplitters and filters to a detector where it is converted into electrical signals. Such spectrometers can be used for optical sensing of the earth. See an article by A. F. H. Goetz et al., Optical Remote Sensing of the Earth, Proceedings of the IEEE, at pp. 966, June 1985.
Conventional imaging spectrometers generally use thin film dichroic beamsplitters and bandpass filters to form the spectral channels of the spectrometer. Relay and focusing lenses are required in each channel to provide means for optimizing performance by maximizing throughput and achieving channel-to-channel coregistration. As the number of channels of a spectrometer increases the array of lenses, beamsplitters and filters that form the channels becomes increasingly complex. These optical elements must be critically aligned with one another within a channel and to an external reference to satisfy performance and coregistration requirements. The optical and mechanical tolerances of these elements must also be tightly controlled to assure that system performance requirements are met. Such spectrometers are typically limited to imaging radiation in the visible and the infrared (IR) regions of light where high spectral resolution is not required, and are unable to image radiation from the ultraviolet (UV) region of light where high spectral resolution is required.
The drawback with conventional spectrometers is that the thin film dichroic beamsplitters and the bandpass filters limit the number of channels which can be formed within a given spectral band because as the number of channels increases, optical crosstalk increases and throughput decreases. In addition, the number of man hours required for aligning and maintaining the optical components may become excessive due to the necessity of critically aligning and focusing the optical components within each channel so as to satisfy system performance requirements while compensating for manufacturing errors that tend to degrade performance. Alignment and focusing of the optical components in one channel affects all of the other channels. The alignment process is interactive, slowly convergent, and may vary from instrument to instrument, making scheduled and required labor hours difficult to estimate. Moreover, serious design difficulties are encountered with conventional spectrometers used in the UV region because the selection of optical materials suitable for beamsplitters and filters at such wavelengths is highly limited.
It is, therefore, an object or this invention to provide a UV imaging spectrometer which eliminates the problems of increased crosstalk and decreased throughput associated with conventional spectrometers, and which is easier to maintain then conventional spectrometers.